It is desired to provide optical switching for optical networks in order to increase speed and usable bandwidth. Various forms of optical switches are known, such as U.S. Pat. No. 4,580,873 in the name of Levinson issued Apr. 8, 1986 to AT&T Bell Laboratories, and U.S. Pat. No. 4,988,157 in the name of Jackel et al. However, these switches have not been commercialized and their capabilities are not yet known. At present, large 1.times.n cross-point optical switches are often configured to obtain functionality provided by nxm matrix switches. In some instances, reliability and cost are reasons for using this "dated" technology in an nxm configuration.
Thus, today, currently available optical switching matrices are being manufactured by use of a single stage architecture where both input and output sides of a P.times.P matrix are comprised of 1.times.P rotary switches such as those described by Duck et al. in U.S. Pat. No. 4,378,144. Configuring a plurality of 1.times.P rotary switches into a single stage P.times.P switch has the following limitations:
a) the P.times.P matrix is not modular and when repairs are required, they must be made to the entire switch; PA1 b) the cost of the switch is largely dependent upon the cost of the number lens-to-fibre units required; and, PA1 c) the maximum reconfiguration time of the component 1.times.P rotary switch is directly dependent upon the dimension of the matrix, and is slow due to the mechanical system. PA1 a waveguide including a first responsive portion and a second responsive portion, each portion having attenuating transmissivity at a signal wavelength in a first state and having transparent transmissivity at a signal wavelength in response to excitement from pump light in a second state; PA1 first pump means optically coupled to a first end of the waveguide for selectively injecting a pump light to excite the first responsive portion; and, PA1 second pump means optically coupled to a second end of the waveguide for selectively injecting a pump light to excite the second responsive portion, PA1 a plurality of first ports for receiving and transmitting optical signals; PA1 a plurality of second ports for receiving and transmitting optical signals; PA1 a plurality of intermediate waveguides for connecting each of the first ports with each of the second ports, each intermediate waveguide having a first responsive portion and a second responsive portion, each portion having attenuating transmissivity at a signal wavelength, and being responsive to excitement from pump light for transparent transmission at a signal wavelength; PA1 a first pump for injecting a pump light for exciting the first portions of the intermediate waveguides; PA1 first switching means for directing the first pump light to intermediate waveguides connected to a selected first port; PA1 second pump for injecting a pump light for exciting the second portions of the intermediate waveguides; PA1 second switching means for directing the second pump light to intermediate waveguides connected to a selected second port, PA1 a first stage comprising a 1.times.(2M-1) optical signal power divider for each input M, wherein M=Ceiling [P/R]; and PA1 a second stage comprising (2M-1) EAND gate matrices, each coupled to outputs of each of the M power dividers, wherein the EAND gate matrices each comprise:
It is usually preferable that optical switches be efficient, fast and compact. As telecommunication networks have evolved over the years and have become more complex, a need has arisen for a matrix switching system capable of optically coupling any one of a large number of other fibers to another.
Furthermore, it is desirable for the switching system to be "non-blocking", i.e. the switching of one input fiber to an output fiber should not interfere with the light transmission of any other input fiber to any other output fiber.
In an application U.S. Ser. No. 08/915,675 now U.S. Pat. No. 5,903,686 by the present applicant, a modular, multiple stage, non-blocking switch architecture is disclosed. This SKOL (4.times.4 matrix switch) architecture provides more efficient structure that makes use of smaller switch units than required for a single stage switch. One disadvantage of the SKOL architecture, however, is that path loss is increased. Each signal must pass through three times more 1.times.N switches than in the SDSR matrix, wherein switches dominate path loss. The SDSR matrix is a single stage switched distribution, switched recombination.
Erbium doped fiber has been used as an optical amplifier as it provides signal amplification when excited ("pumped") by a laser preferably with light of a different frequency from the transmitted signal. Matrix switching devices experience loss from passive distributors that divide signal energy, and from active selectors which also introduce losses. Erbium doped fiber amplifiers have been used in matrix switches to compensate for the losses.
When an erbium doped fiber or waveguide is not "pumped," it causes attenuation at the signal wavelength. The level of dopant in the fiber, and the length of the fiber determine the amount of gain or attenuation experienced by the signal.
In U.S. Pat. No. 5,570,218 issued to Sotom, Oct. 29, 1996, in the name of Alcatel N.V., use is disclosed of erbium doped waveguides in an optical switching matrix to reduce crosstalk. The Sotom switching matrix comprises an input waveguide coupled to a 1.times.n splitter that distributes a signal into erbium fiber amplifiers for distribution to each of the output ports. One amplifying fiber of the splitter is selectively coupled to an output waveguide by an active selector. The active selector is a switch that permits pump light to be transmitted from a single laser pump through the selected amplifying fiber to amplify a signal transmitted through the same fiber. The signal is also switched by the active selector to the selected output port. Other signals from the 1.times.n splitter are not amplified. As a result the selected signal is significantly stronger than unwanted signals which may cause crosstalk.
This switch is not entirely satisfactory, however. The optical selector must be able to operate at both pump and signal wavelengths, which is a difficult requirement for many optical selector technologies. The number of erbium doped amplifiers requi red is equal to the square of the number of switch points. Since these amplifiers are not in general, inexpensive, it would be advantageous to construct a large switch out of smaller subunits.
It is shown here that the number of amplifiers required can be fewer than are taught in the Sotom design, if the subunits are blocking switches. The use of a blocking switch subunit, in the present invention, enables a switch construction to make use of a simple power divider to broadcast of one of the input signals to many output ports.
It is desired to provide an optical switch providing passive optical switching for a signal without having active switch elements in the signal path.
It is further desired to provide a fully blocking optical switch element to prevent crosstalk.